Benzene and toluene gas-phase photocatalytic degradation over H2O and HCL pretreated TiO2: by-products and mechanisms

Photocatalytic oxidations of toluene and benzene in air were carried out ov er water (TiO2/H2O) and HCl (TiO2/HCl) pretreated TiO2 as this latter pretr eatment enhances the toluene removal rate. The main purpose was to identify intermediate products. No gas-phase byproducts were detected by direct GC/ FID analysis under our conditions despite the high aromatic concentration ( 50 mg m(-3)) and the short contact time (ms); this result illustrates an at tractive capability of this air purification method. Adsorbed intermediate products recovered by extraction from the used photocatalysts were the same over TiO2/H2O and TiO2/HCl; i.e. no chlorinated products were found in thi s latter case. Benzoic acid, benzaldehyde, and benzyl alcohol were three ma jor toluene intermediate products identified; trace derivatives of these pr oducts and of toluene that were monohydroxylated on the ring were also dete cted in catalyst extract samples. The benzene major by product was phenol w hich was accompanied by hydroquinone and 1,4-benzoquinone. Acetic and formi c acids were also formed from both benzene and toluene. For both aromatics, a water extraction of the used photocatalysts permitted separation of a ye llow viscous material that settled between water and TiO2 after centrifugat ion. We have not identified the products contained in this material but bel ieve them to be polymeric products which may be at the origin of the decrea sed photocatalytic activity of used TiO2 with respect to fresh TiO2. Finall y, we discuss the photocatalytic oxidation pathways. (C) 1998 Elsevier Scie nce S.A. All rights reserved.